Titration calorimetry of surfactant-membrane partitioning and membrane solubilization

The interaction of surfactants with membranes has been difficult to monitor since most detergents are small organic molecules without spectroscopic markers. The development of high sensitivity isothermal titration calorimetry (ITC) has changed this situation distinctly. The insertion of a detergent into the bilayer membrane is generally accompanied by a consumption or release of heat which can be measured fast and reliably with modern titration calorimeters. It is possible to determine the full set of thermodynamic parameters, i.e., the partitioning enthalpy, the partitioning isotherm, the partition coefficient, the free energy, and the entropy of transfer. The application of ITC to the following problems is described: (i) measurement of the critical micellar concentration (CMC) of pure detergent solutions; (ii) analysis of surfactant-membrane partitioning equilibria, including asymmetric insertion; and (iii) membrane-surfactant phase diagrams. Finally, the thermodynamic parameters derived for non-ionic detergents are discussed and the affinity for micelle formation is compared with membrane incorporation.